Just something random to check - make sure autologin is disabled (this tells you how to turn it on, but it's easy enough to reverse the process):
http://support.microsoft.com/default...b;en-us;315231

If you need to reset your password, you can always use the Offline NT Password & Registry Editor. You might lose access to encrypted files and other user-related stuff, however.

My best guess from what you're describing is that someone set up a password for you, and set it to automatically log in with that username and password. It's either that or your user profile/registry somehow got corrupted.